Mansions Phase 1-CS 971021 Morrison Hydrology Engineering, Inc.
924 Blossomwood Ct, Arlington, Texas 76017
Telephone (817) 465-7882 Mobile (817) 980-3819 Pager (8~7) 630-9496
October 21, 1997
Mr. Kenneth M. Griffin, P.E.
Assistant City, Manager
City Engineer
Cit3' of Coppell
255 Parkway Blvd.
P.O. Box 478
Coppell, Texas 75019
Re: Review, of:
Application for Conditional Letter of Map Revision for
Mansions on the Lake
Denton Creek, City of Coppell Texas
Dear Mr. Griffin:
As you are aware Kimley Horn and Associates as the City Floodplain review consultant has asked us
assist them in review of the subject study'. The following is a summary of the results of this review':
1. Federal Emergency Management .4gency (FE.~4) effective model duplication.
In order to duplicate a FEMA effective model it is normally required to compute and print all the
sections duplicated in order to determine if the model is actually duplicated. The study suppressed
the printout of the first 28 sections. In addition it is usually appropriate to include any
correspondance with the FEMA library to verily' their comments.
We obtained a printout from FEMA as the effective model a couple of years ago. It does not
match your duplication. We are axvare that Weir and Associates was working on another
submittal to FEMA in conjunction with improvements at Denton Tap Road. This may have
resulted in a revised effective model. Without documentation it is not possible to veri~.' this.
Please provide your correspondence with FEMA or better documentation to veri~ the duplication
model.
2. Proposed condition model
In order to reclaim the floodplain area it was necessaxy to offset the decreased conveyance, which
results from filling a portion of the floodplain. This was accomplished ~' creating a pond and
some additional grading. The assumption in the model was that full conveyance was
accomplished 6 feet below the constant pool level in the pond. Technically, some conveyance can
be a~umed below- water level, but we do not believe full conveyance can be assumed for 6 feet.
To be conservative it is best to assume no conveyance below the pond water level. However, we
would be open to technical justification for "below water" conve~nce.
Please provide technical justification for conveyance below water levels or adjust the model so that
the GR cards do not extend below the water level (or no more than a couple of feet below the
water level).
Morrison Hydrology Engineering, Inc.
924 Blossomwood Ct. Arlington, Texas 76017
Telephone (817) 465-7882 Mobile (817) 980-3819 Pager (817) 630-9496
3. FE,~ Forms
MT2 Form I Page 2
9. The state has no jurisdiction over the floodway.
10. (1 A) This revision request involves development in the floodway. Please refer to
definition of development in the FEMA regulations and city ordinance. This includes
Pond and grading.
15. Pond and grading is used to offset increases in flooding, and as such is considered a flood
control structure. This requires modification of the form to address maintenance.
MT2 Form 4
2. Area studied by detailed methods from section 18440-56940.
6. FEMA requires tie-in within .05 feet. This will require that the model be extended
upstream.
Water Surface Elevation Check should extend upstream until the existing and proposed water
surface elevations match, or the end of the study is reached.
In our discu~ions with Mr. Michael Martin, P.E. with the city staff it is our understanding that this
project is not in the Corridor Development Certificate area so we did not review any of the
requirements in regard to this.
Based on our review we would recommend that the information above be investigated before approval
of th is study.
Sjnc~erely,
Ronald W. Morrison, P.E., C.F.M.
Senior Hydrologist